1st off grab a pen paper and tape measure angle protractor etc, and measure frame before doing anything, this will tell what your starting point is.
Cut at red lines, grind paint off 3 inches back, deburr inside holes. Make four 4 inch long slugs to go inside tubes (solid or tube) .
Now drill two 1/4 inch dia holes 1 inch back on all tubes. Now drill two 1/4 inch dia hole 1-1/2 inches back, 90* from 1st set.
You now have 32 holes drilled, deburr them so the slugs still go into tubes.
Use a sharpy and draw a line around slugs in the middle, you should have about a 1/16 line.
Insert a slug until the marker line just touches tube, now you have 2 inches sticking out.
Plug weld one hole, now repeat on other 3 legs on one half of cart DO NOT WELD ALL 16 HOLES, you should now have 4 holes welded and four plugs in.
Now slid front 4 frame ends onto the slugs, use a penny ~1/16 of an inch as a spacer to set gap and check with tap measure for square and plumb.
Plug weld 1 hole of those four frame parts.
Now frame is tacked together, dbl check squareness etc, finish plug welding each hole, alternate between all 8 ends, when done burn in the 8 tube ends .
A flat surface to clamp bottom down before welding anything would really help
The top tube will really help with flex issues 1 rail carts would have
